Mast cells are the cellular nidus of allergic diseases and the cell responsible for disease in urticaria pigmentosa and systemic mastocytosis. This project is designed to ablate mast cells by attaching cytotoxic agents to IgE or antigen and selectively indroducing the toxic product into mast cells. IgE linked to ricin kills RBL cells in vitor spontaneously or after cross-linking the IgE with anti-IgE. IgE linked to ricin's A chain kills in the presence of monesin, a carboxylic ionophore, or in the absence of monensin at higher concentrations. IgE-ricin A conjugates injected intradermally significantly lower rat cutaneous histamine levels in vivo suggesting that this agent might work in vivo as well.